Rotary speed sensors of the kind of interest here are used in vehicle technology to obtain one of the most important input variables for the electronic control of a vehicle. Thus, the conventional antilock brake systems (ABS), antislip controls (ARS) and vehicle stability control systems (FSR, ASMS) are forced to rely on permanent measurement and evaluation of the rotary speed response of the individual vehicle wheels. The rotary speed signals of each individual vehicle wheel are recorded and transmitted via a sensor line to an electronic control unit, for the purpose of being evaluated.
Rotary speed sensors exist of very different types of construction and working capacity. It is true that rotary speed sensors installed in vehicles are made up basically of an encoder rotating together with a wheel in form of a toothed disk, perforated disk, or the like, and a stationary measurement detector. For reasons of technology and cost, inductive sensors or measurement detectors were preferred up till now, in which the encoder generates an alternating signal that corresponds to the rotary motion of the vehicle's wheel. The frequency of the alternating signal is evaluated in different ways to obtain the rotary speed data.
Currently, so-called passive rotary speed sensors have the greatest distribution, which work according to the reluctance principle. These rotary speed sensors have as measurement detector a copper coil having a permanent magnet. The measurement detector is coupled magnetically to the recording disk used as the measurement transducer or to another encoder. The encoder modulates the magnetic switch reluctance in synchronous motion, an alternating voltage representing the motion being induced in the copper coil, whose frequency may be evaluated as measured quantity for ascertaining the rotary wheel speed. The magnitude of the induced signal voltage is a function of the rotary speed and of the air gap between the measurement transducer and the measurement detector, or between the toothing and the rotary speed sensor.
Besides that, so-called active rotary speed sensors are also generally known, to which the subject matter of the present invention relates. Active rotary speed sensors are made up in principle of the combination of a magnetostatically sensitive element having a permanent magnet, which is magnetically coupled to the encoder. Here too, the encoder modulates synchronous in motion the magnetic switch reluctance or the field direction, the sensor element reacting with the change in the flux density or to the motion of a field vector. Known examples of such magnetostatically sensitive elements are Hall probes and magnetoresistive structures based on permalloy alloys. The magnitude of the signal voltage at the rotary speed sensor is a function of the air gap, but is independent of the rotary speed and the frequency. Active rotary speed sensors are therefore gaining more and more importance.
An active rotary speed sensor of this type is known from International Published PCT Application No. 95 17680. The fixed part of the rotary speed sensor includes a magnetoresistive sensor element having a permanent magnet that is used as booster magnet, and besides that has electronic circuit components for signal conditioning. An active rotary speed sensor requires a current supply. The output signal of the active rotary speed sensor is a binary current signal, which is composed of impressed currents of different amplitudes. The rotary speed information is located in the frequency or in the change between the two current levels. Known rotary speed sensors of this type give rise to a square wave signal, whose frequency reproduces the measured rotary speed.
For the further processing of the information supplied by the active rotary speed sensor, a microcontroller is usually inserted, on whose input side the circuit for the signal conditioning is preconnected. A passive rotary speed sensor also requires a signal conditioning circuit for connection to a microcontroller as the electronic control unit for additional signal processing of the rotary speed signal. Since the signal conditioning circuit for the connection of an active rotary speed sensor is constructed in a different manner from the signal conditioning circuit for the connection of a passive rotary speed sensor, the sensor type determines the type of signal conditioning circuit that is to be used.